Electrolytic method for preparing dialkyl dicarboxylates



April 11, 1967 TSUTOMU KUWATA ETAL 3,313,717

ELECTROLYTIC METHOD FOR PREPARING DIALKYL DICARBOXYLATES Filed Dec. 11,1962 INVENTORS Tsu oM u KuwA'r -1 KAW BY A A TTORNE) United States PPatented Apfj li ll fi 3,313,717 ELECIRDLYTKC METHQD FOR PREPARINGDIALKYL DXCARBQXYLATES Tsutornu Kuwaiti and Sadao Yoshikawa, Tokyo,Japan, assignors to Soda Koryo Kabushiki Kaisha, Tokyo, Japan, acorporation of Japan Filed Dec. 11, 1962, Ser. No. 243,830 Claimspriority, application .lapm, Sept. 17, 1962, 37/39,?51 6 Claims. (Cl.204-79) The present invention relates to a method of preparing dialkyldicarboxylates and more particularly, to a method of synthesizingdialkyl dicarboxylates by means of electrolytic oxidation in which therehas been utilized an anodic reaction.

More specifically, this invention relates to a method of preparingdialkyl dicarboxylates which comprises forming dieste'rs (dialkyldicarboxylates) by the electrolytic oxidation of an alkali salt solutionof an alkyl hydrogen dicarboxylate, etiecting the electrolytic oxidationin a cell consisting of at least one cathodic compartment and one anodiccompartment, feeding to said anodic compartment a neutral or slightlyacidic solution containing the aforesaid alkali salt, and passing acurrent of electricity while maintaining said cathodic compartment in analkaline state.

Prior to the present invention it was known that dialkyl dicarboxylates,e.g., dlmethylthapsiate could be prepared by electrolytic oxidation ofthe alkali salts of alkyl hydrogen dicarboxylate, for example, thesodium salt of methyl hydrogen azelate, using a platinum wire as theanode in accordance with the following Equations 1, 2 and 3, namely:

However, in accompaniment with the principal reactions in accordancewith the foregoing Equations 1, 2 and 3, there occur side reactions inaccordance with the following Equations 4, 5, 6, 7 and 8, namely:

As a result, the by-products such as methyl octenate [Equation 5],methyl caprylate [Equation 6], methyl oxycaprylate [Equation 7] andmethyl-w-oxocaprylate [Equation 8] are formed.

Therefore, in the preparation of diesters in accordance with Equations1, 2 and 3, diesters cannot be obtained in good yield unless the sidereactions of Equations 4, 5, 6, 7 and 8 are restrained. When restrainingsuch side reactions, it was known in the prior art to raise theconcentration of the material alkyl hydrogen dicarboxylate, to operateat a high current density, and when restraining the reaction of Equation7 to lower the pH of the solution of the material alkali salt of alkylhydrogen dicarboxylate.

However, what becomes a problem with respect to the pH of the liquid isthat in this reaction the alkali is formed at the cathode, and unlesssome means or other could be adopted, the pH of the liquid raises highas the reaction proceeds and the rate of formation of oxy acids becomeshigh. In addition, the saponification of the material alkyl hydrogendicarboxylate occurs so as to bring about a fall in the yield of themain reaction product.

As for methods that have been known hitherto for restraining said sidereactions, one such method involves the use of an alkali salt of alkylhydrogen dicarboxylate in a methyl alcohol solution as illustrated inthe Journal of Chemical Soc., 1679 (1938) and 3326 (1950). Still anothermethod utilized an aqueous solution of a potassium salt prepared byusing, based on the alkyl hydrogen dicarboxylate, potassium hydroxide inabout its stoichiornetric quantity, neutralizing the potassium hydroxidethat forms at the cathode, as the reaction proceeds, to prevent the pHof the reaction liquid to move to the alkaline side by replenishing thestarting material alkyl hydrogen dicarboxylate monoester, Whileseparating the diesters formed.

However, results that will give complete satisfaction commerciallycannot be obtained even by these methods. More particularly, thepreparation selectively and advantageously in good yield continuouslyover a long period of time is impossible.

In accordance with these conventional methods, as there are provided acathode and an anode in a single electrolytic cell not partitioned by abulkhead, the material monoester (alkyl hydrogen dicarboxylate), and theesters which are formed as the pro-duct oil, being hydrolyzed by thealkali forming in the vicinity of the cathode form alkali salts or"acids. The surface activity of the material liquid containing thesealkali salts becomes intense to a marked degree so as to accelerate thesolubilization of the neutral oil product whereby said oil accumulatesin the reaction liquid. The quantity that accumulates increases with thepassage of time, and if the reaction is continued this brings about anincrease in the electrical resistance. On the other hand, as a result ofsecondary reactions between the alkali salts of acids formed by thehydrolysis of the thus formed esters and particularly between thesesalts and the alkali salt of the material monoester, various troublesomelay-products besides those by the aforementioned Equations 48 areformed, which bring about the degradation of the intended diestercompositions that are formed. Considerable quantities of high boilingfractions and resinous substances are formed so as to greatly impede thecontinuance of the reaction and in extreme cases render operationsimpossible.

Moreover, in the case when methyl alcohol is used as the reactionsolvent in accordance with the conventional method, since the reactionis started with a liquid of high concentration from a zone of low pH andends with a liquid of low concentration and a higher pH (slightlyalkaline) than at the start and in a state in which the product formedis accumulated in the material liquid, the carrying out of the reactionunder constant conditions is not possible. In other words, in order tomaintain the electric current at a level necessary for proceeding withthe reaction, concomitant with the increase in resistance due to theaccumulation of the product formed the voltage must be graduallyincreased. Furthermore, even if the voltage is so regulated, thereaction must still be discontinued because .of the rise in the pH andthe increase in the resistance. In addition, the accumulation of theformed product in the liquid inevitably makes its separa tion operationdifficult of performing. Hence, it is of great disadvantage from acommercial standpoint.

Also, in the case of the conventional method in which the pH ismaintained at the desired pH by replenishing the alkyl hydrogendicarboxylate as the reaction proceeds, if continuous operation over anextended period is attempted, the formation of the intended diestersgradually falls. Therefore, when attempting to obtain diesters inamounts equaling that obtained at the start of the reaction, it becomesnecessary to continue the reaction for an exceedingly long period oftime, which would make it impractical. Consequently, the intendeddiesters cannot be obtained regularly by such a method.

Again, since a very high voltage is required in these conventionalmethods, the electric power consumption (currentxvoltage) is very great.Hence, they are at a disadvantage commercially and their continuousoperation actually is impossible.

As a result of research conducted for a method of preparingadvantageously and effectively on a commercial scale, by eliminating thedisadvantages of the conventional prior art methods, diesters fromalkali solutions of alkali salts of an alkyl hydrogen dicarboxylate bymeans of electrolytic oxidation, it was noted that the foregoingelectrolytic oxidation is an anodic reaction, and that in theconventional prior art methods, studies were made only with respect tothe reaction mechanism and the operational conditions at the anode.However, the effects that the cathode had on the anodic reaction wereoverlooked by the prior-art Workers in the field. As a result of muchstudy regarding these effects, it was found that unless the reaction wascarried out in a state in which the effects of the cathode arecompletely excluded, satisfactory results could not possibly beobtained, and that when carrying out the anodic reaction in this state,in which the effects of the cathode are excluded, it was necessary toeffect the reaction while isolating the anodic compartment from thecathodic compartment. In addition, it was also found that acation-exchange membrane should be used as such isolating means.Moreover, it was found that the objects of the present invention couldnot be obtained by other isolating means such as, for example,perforated membrane, unglazed pottery, sulphate paper, etc.

It was also found that if the reaction liquid was fed to the anodiccompartment and alkali was added to the cathodic compartment so that thecathodic compartment liquid would become alkaline at the time of thestart of the electrolysis, then it would be sufficient if Water was fedthereto to maintain said alkalinity. Thus, the herein describedtroublesome side reactions in the vicinity of the cathode as well as theside reactions which brought about the other defects of the conventionalmethods could be essentially overcome.

It was also found that by means of the method of the present inventionit was made possible for the first time to obtain the intended diestersin a regular manner and that a continuous operation could be carried outthat would be satisfactory on a commercial scale. It was further 4-found that since the resistance of the liquid becomes exceedingly smallas compared with the conventional methods, the reaction could be carriedout at a low voltage which is advantageous commercially accordingly, there- 5 action could be carried out with marked advantage particularly incase of a continuous operation, and the power consumption could bereduced considerably.

It was also found that since there was no need for complicated andimperfect control means for controlling the alkali accumulation whichformation was inevitable in the conventional methods the pH of thereaction liquid could be controlled easily and satisfactorily.

Moreover, a variation in the pH of the anodic liquid in the anodiccompartment between the vicinity of the 5 inlet and the vicinity of theoutlet therefor could hardly be observed; instead, it was found that thepH would shift about 0.05 towards the acidic side which was favorablefor the reaction and that the pH would shift about 0.2 towards thealkaline side which was unfavorable for the reaction when the membranewas not present. It was also found that since different solvents couldbe used in the cathodic compartment and the anodic compartment, thetransmissibility of electricity could be enhanced, and also that it waspossible to fix the substance that sets up impeding actions in theanodic compartment and in the cathodic compartment and thus prevent theoccurrence of impeding actions.

Accordingly, it is an object of the present invention to provide acommercially advantageous and effective method of preparing in goodyield and with regularity dialkyl dicarboxylates with low voltage andelectric power while preventing the occurrence of undesirable sidereactions. n Another object of the invention is to provide a method 05of preparing dialkyl dicarboxylates in which the defects of theconventional methods can be overcome and in which a continuous operationwith regularity can be carried out satisfactorily over an extendedperiod of time without the need for complicated operating means.

Other objects and advantages of this invention will be apparent from thedescription which follows.

According to the method of the present invention, cation-exchangemembranes are used as partitions and at least one each of an anodiccompartment and a cathodic compartment is constituted. To this anodiccompartment a solution of an alkali salt of an alkyl hydrogendicarboxylate Whose pH has been adjusted to the vicinity of neutral orslightly acidic side is fed and while maintaining the cathodiccompartment in an alkaline state electricity is passed. In the firstplace, when considered from the nature of this type of electrolyticoxidation reaction, while it is one of the requisites that the pH of thereaction liquid does not become alkaline as the reaction proceeds,according to the method of this invention, as shown in FIG. 1,concomitant with the electric discharge of R-COO (wherein R is an alkylgroup), the potassium ion K+, for example, passes through thecationexchange membrane 1 and moves to the cathode to form -KOI-I. Onthe other hand, on the anode the carboxyl 6O radical loses a molecule ofCO to produce an alkyl radical as in the foregoing Equation 2 and formsdiesters as in Equation 3.

Moreover, since the OH of the cathodic compartment cannot enter theanodic compartment because the parti- 5 tion is a cation-exchangemembrane, it can restrain the formation of oxy acids, the by-productsresulting from the reaction of Equation 7.

With respect to the alkali salt of an alkyl hydrogen dicarboxylate thatis used in this invention, compounds in which the monoester is composedof a dicarboxylic acid of 3-13 carbon atoms and a lower aliphaticalcohol of 1-3 carbon atoms and said alkali salt is an alkali metalsalt, particularly a Na salt or a K salt, are satisfactorily used.Included as such dicarboxylic acids are, for example, the malonic,succinic, Z-methyl succinic,

glutaric, adipic, pi-melic, suberic, azelaic, sebacic and brassylicacids. Also the lower aliphatic alcohols for the esters thereof includemethyl alcohol, ethyl alcohol and propyl alcohol (normal and iso).

Accordingly, the alkali salts of alkyl hydrogen dicarboxylates as usedin the present invention include the Na and K salts of the alkylhydrogen dicarboxyiate such as, for example, methyl hydrogen malonate,ethyl hydrogen malonate, methyl hydrogen adipate, ethyl hydrogenadipate, propyl (n-, iso-) hydrogen adipate, methyl hydrogen azelate,ethyl hydrogen azelate, propyl (11-, iso-) hydrogen azelate, methylhydrogen sebacate, ethyl hydrogen sebacate, propyl (11-, iso-) hydrogensebacate, methyl hydrogen pimelate, ethyl hydrogen pimelate, propylhydrogen pimelate, methyl hydrogen suberate, ethyl hydrogen suberate,propyl hydrogen suberate, methyl hydrogen brassylate, ethyl hydrogenbrassylate, methyl hydrogen-Z-methyl succinate, ethyl hydrogen Lmethylsuccinate, etc. These compounds may either be used singly or twodifferent compounds may be combined and used. If difierent monoesters ofalcohols are used, a diester will of course be obtained whose esterportions at the two ends will be different as in Equation 3, forexample, methyl propyl sebacate.

Thus, according to the method of this invention, the diesters which canbe prepared with advantage include, for example, dirnethyl succinate,diethyl succinate, methdrogen sebacate, propyl (n-, iso-) hydrogensebacate, dipropyl (n-, iso-) sebacate, methyl propyl (n-, iso-)sebacate, dimethyl brassylate, diethyl brassylate, dimethyln-tridecane-l:13-dicarboxylate, diethyl n-tridecanelzl3-dicarboxylate,dimethyl thapsiate, diethyl thapsiate, methyl ethyl thapsiate, dimethyln-hexadecane-lzlo-dicarboxylate, diethyln-heXadecane-l:l6-dicarboxylate, dimethyl 3-methyl-n-tridecanedicarboxylate, diethyl 3- methyl-n-tridecane dicarboxylate, etc.

In accordance with the method of the present invention when preparing aneutral or slightly acidic solution of the alkali salts of an alkylhydrogen dicarboxylate such as described hereinbefore, the salts may bedissolved in an aqueous solution or a water-water soluble loweraliphatic alcohol mixture and the pH adjusted with an alkyl hydrogendicarboxylate, an alkali, and water or a water-water-soluble loweraliphatic alcohol mixture may be mixed so as to obtain the desiredconcentration and pH. The alkali used in this instance include thosewhich can form a sodium or a potassium salt such as, for example,caustic soda, caustic potassium, sodinm carbonate, sodium bicarbonate,potassium carbonate, potassium bicarbonate, etc. And as thewater-soluble lower aliphatic alcohols, included are, for example,methyl alcohol, ethyl alcohol, propyl alcohol (n-, iso-), butyl alco hol(n-, iso-, tert-), etc. In case a water-water-soluble lower aliphaticalcohol mixture is used, if the concentration of the alcohol is high, aminute amount of the alcohol passes through the partitioning membraneand moves into the cathodic cell where it disperses. To compensate forthis portion the anodic liquid is suitably replenished with a minuteamount of alcohol.

For effecting the isolation of the anodic compartment from the cathodiccompartment, which is an important feature of the present invention, acationexchange membrane is used.

In this connection a cation-exchange membrane which is either ahomogeneous membrane, a heterogeneous membrane, or a united membrane maybe used; for example, strongly acid cation-exchange resin membranes suchas the polystyrene sulfonic acid resin membrane, the styrene sulfonicacid-butadiene type resin membrane, the polyethylene-styrene sulfonicacid type resin membrane, the interpolymeric resin membrane, etc. Theweakly acid cation-exchange resin membranes such as the divinyl benzeneacrylate, the divinyl benzene methacrylate and the divinyl benzenemaleic anhydride types may also be used. According to this invention,while at least one each of an anodic and cathodic compartment comprisingat least one cation-exchange membrane, in commercial practice it isadvantageous to adopt a multicellular arrangement, as shown in FIG. 2,by using at least two or more cation-exchange membranes 1, 1', 1"

to form at least three or more compartments wherein the two endcompartments are made cathodic and, the compartments adjoining theretoanodic and thereby forming in alternation cathodic and anodiccompartments. It is of course possible to make the compartments at theirends anodic, but since it is advantageous to use platinum as the anodes,and since only one surface of an expensive plate would be utilized, thecompartments at the two ends should be made cathodic.

To the thus constituted anodic cells is fed a neutral or slightly acidicsolution containing an alkali salt of an alkyl hydrogen dicarboxylate,is fed to the anodic compartment and, the solution used as aconcentration of 2040% by weight, preferably Z535% by weight, and a pHof preferably 6-7. ()n the other hand, the cathodic compartments aremaintained in an alkaline state. Since allralis are formed the cathodiccompartments by the alkali metal ions which nave moved over from theanodic compartment by passing through the cationexchange membranes, thealkalinity of the former may be maintained merely by adding water duringthe continuance of the reaction to adjust the alkaline concentrationpreferably to 0.53% by weight. Such an alkali may be, for example,caustic soda, potassium hydroxide, sodium carbonate, sodium bicarbonate,potassium carbonate and potassiun bicarbonate. The akali metal of thealkaline substance that is added to the cathodic compartment at thestart may be the same or different from the alkali metal that is derivedfrom the alkali salt of an alkyl hydrogen dicarboxylate of the anodiccells. drogen dicarboxylate of the anodic compartment.

The object of the present invention are satisfactorily attained bypassing an electric current described above and carrying out theelectrolytic oxidation. While the electric current used will be varieddepending on the scale of the electrolytic compartments, the reactiontemperature of the electrolytic oxidation of the anodic compartment, theconcentration of the anodic liquid, the kind and dimensions of thepartitioning membrane, etc., the method of this invention can be carriedout continually over an extended period of time regularly at a lowvoltage and low power requirements which are of great advantage over theconventional methods. For example, in a hour operation for obtainingdimethyl thapsiate from an aqueous solution of a potassium salt ofmethyl hydrogen azelate, the voltage and power required by theconventional methods were on the order of 12.3 volts and l2.4 37.7 kWh.(increasingly become greater as the reaction proceeded) per kg. ofdimethyl thapsiate, and furthermore, as already mentioned hereinbefore,numerous technical difiiculties were encountered. In contrast, themethod of the present invention can be carried out satisfactorily by theuse of a voltage of 9 volts and a consumption of power on the order of4.62 kwh. (practically constant with the passage of time) per kg. ofdimethyl thapsiate produced when operating for the same number of hoursunder identical conditions, the technical difiiculties of theconventional methods being moreover surmounted.

Either a batch or a continuous process may be applied to the practice ofthe method of this invention. In the conventional methods the continuousoperation over an extended period on a regular basis was substantiallyimpossible, but in accordance with the present invention it is possibleto operate continuously over an extended period very advantageously andeffectively, and especially when the continuous operation is carried outusing the aforesaid multicellular system. Such a continuous method maybe carried out by feeding continuously, while passing an electriccurrent, a neutral or slightly acidic solution containing an alkali saltof an alkyl hydrogen dicarboxylate to the anodic compartment and waterto the alkaline solution in the cathodic compartment, taking out on theother hand the liquid containing the formed diesters from which thesaid-diesters are separated, then after deplenishing the residual liquidwith the materials consumed and adjusting the pH in concomitancetherewith, recycling said liquid again to the anodic compartment, Whiledischarging continuously the cathodic compartment liquid.

In order to take out continuously the oil produced comprising mainlydiesters which floats normally at the top of the anodic compartment, anymeans by which the floating oil can be continuously taken out as quicklyas possible can be utilized. However, from the operational standpointthe removal by means of, for example, the overflow method is convenient.On the other hand, when the liquid phase containing the diesterscollects at the bottom (on account of its pH or specific gravity) of theoil formed, it can be taken out by providing a suitable outlet at thebottom. When using a water and water-soluble lower aliphatic alcoholmixture, if the concentration of the alcohol is relatively high, theresulting oil containing the diesters formed dissolves in the liquid andbecomes in its entirety a liquid phase containing the diesters formed.In this case, the extraction and separation may be per formed by using amethanol-insoluble lower aliphatic hydrocarbons, 'for example, hexane.When the concentration of the alcohol is relatively low, for example,about 10% by weight, such a necessity does not arise. The ad justment ofthe concentration and pH of the residual liquid may he carried out byadding an alkyl hydrogen dicarboxylate and an alkaline substance, or itcan also be accomplished by the addition of an alkali salt of an alkylhydrogen dicarboxylate and a small amount of an alkvl hydrogendicarboxylate.

In order to illustrate the invention further, the following examples aregiven, it being understood that these are intended to be ierelyillustrative of the invention and not in limitation thereof.

A. The construction the elcctrolyr Two sheets cf cation-exchangemembranes were used as partitions by which the electrolyzer was dividedinto 3 isolated compartments, of which the compartments at the two endswere made the cathodic compartments and the middle Was made the 'anodiccompartment.

Cathodic compartment:

Length-20 cm. Height29 cm. Width-52 cm. Inside capacity-3 liters Fittedat the bottom with an inlet for water and at the top with an overflowoutlet. Anodic compartment:

Length-20 cm. Height29 cm. Width2.6 cm. Inside capacity-1.5 litersFitted with a glass tube, a cooler, an inlet for the anodic liquid atthe bottom and an overflow outlet at the top. Cation-exchange membrane:

Effective area-20 cm. 20 cm., 400 cm. Selemion CMG-lO styrene sulfonicacid-butadiene type (produced by Asahi Glass Kabushiki Kaisha, Japan).

Thickness-0.2l-0.25 mm. Specific resistance 300Q-cm. Transference numberTNa+ 0.91 Anode: Platinum Wire 0.5 mm. diameter, 159 cm. long,

surface area 25 cm 8 Cathode: Thin plate of nickel, stainless steel oriron 19 cm. 20 cm., 380 cm.

B. Operation As shown in the flow sheet of FIG. 3, one example comprisespreparing by means of the material tank 1 and the adjusting tank 2 aneutral or slightly acidic solution containing in a suitableconcentration an alkali salt of alkyl hydrogen dicarboxylate, whichisused as the anodic compartments liquid of the electrolytic reactionvessel 3, and on the other hand, filling the cathodic compartments ofthe electrolytic reaction 3 at first with a dilute aqueous alkalinesolution containing a small amount of an alkali, and then passing anelectric current. An anodic liquid is introduced into the anodiccompartment from the bottom thereof at the rate of 3-5 l./hr. from thematerial tank 1. The oil produced containing the intended diesters whichrises to the surface of the anodic compartment liquid along with theevolution of carbon dioxide that attends the progress of the reaction iscaused to flow out from the overflow outlet at the top and is conductedto an oil and water separation tank 4. The diesters which have beenseparated here are conveyed to a receiving tank 5, whereas the residualliquid after having its concentration and pH adjusted is recycled to theelectrolytic reaction 'vessel 3 via the material tank 1. Since theanodic and cathodic compartments are separated by means of anion-exchange membrane, when left as it is without any countermeasuresbeing taken the concentration of alkali of the cathodic solution becomeshigher as the reaction proceeds resulting in a decrease in the selectivepermeability of the membrane so that the adjustment of the pH becomesdifficult of accomplishment. To prevent this, water is fed to thecathodic compartment to dilute the cathodic compartment liquid and thusmaintain the concentration of alkali at 0.53%. In this case, thealkaline water which flow out from the cathodic compartment is recoveredat the outside of the vessel.

The oil of the receiving tank 5 is adjusted to pH 9 by means of acaustic alkali solution, extracted with a solvents using aliphatic andaromatic hydrocarbons, preferably such as benzene, toluene, hexane,etc., after which the monoesters mixed in said oil is separated as anaqueous solution of alkali salt, the extract washed with water, andafter recovering the solvent followed by dehydration, rectification iseffected to obtain the intended purified diesters.

Example 1.Using 3,200 g. of methyl hydrogen azelate and 1,230 g. ofpotassium carbonate an aqueous solution having a concentration of 30%and a pH of 7 was prepared, and by operating as described in section B,above, with the electrolyzer described in A, above, employing a directcurrent of 9 volts and 20 amperes dimethyl thapsiate was obtained.

During the operation the electrical resistance was substantiallyconstant throughout the reaction which was carried out in a regularmanner. After operating for hours, the consumption of monoesters was13,600 g. and the dimethyl thapsiate obtained was 6,222 g.

When the results of the foregoing electrolytic oxidation, according tothe method of the present invention, extending over 160 hours from thestart of the reaction was divided into five sections comprising sectionI (20 hours), section II (35 hours), section III (35 hours), section IV(35 hours) and section V (35 hours), and the reaction conditions,composition of the oil formed, and the changes in amount of oil formedper unit electric current by sections were investigated, they were asshown in Table I, below. The oil formed, as shown in the table, however,has been shown in this instance by the oil distilled off duringrectification.

TABLE I Section I Section II Section III Section IV Section V TotalMaterial:

Material monoester (g) 3, 200 Material monoester replenished (g) 1, 820Reaction Conditions:

Reaction time (hr.) 20 Amount of electric current (ah)- 400 Currentdensity (a./dm.'- 80 Oil Formed:

Rate of formation (g./ah.) 2. 90 3. 2. 99 2. 99 2. 97 Oil formed (g) l-1, 160 2, 100 2, 090 2, 094 2, 080 9, 524 Methyl-7-octenoate (g) 50-70"C mm. 5 186 (16.0%) .557 (17.0%) 341 (16.3%) 355 (16.0%) 349 (16.8%) 1,568 Methyl-S-oxyoctanoate (g) 120 C./5 mm.

Hg". 222 (19. 0%) (20. 0%) 370 (17. 7%) 387 (18. 5%) 337 (16. 2%) 1, 734Dimethyl thapsiate (g) 180-185 C./3

Hg. 755 (65. 0%) 1, 320 (63. 0%) 1, 379 (66. 0%) 1, 372 (65. 5%) 1, 394(67. 0%) 6, 222 High boiling fractions Yield:

Monoester consumed (g.) Oil iormed/Monoesters consumed (p .nt) Dimetjhylthapsiate/monoester consumed (pen cent Yield (percent of theory)- "I*Indicates the principal distillation range of each of the fractions.

On the other hand, when, for purpose of comparison, the same experimentwas repeated with the operation being continued for 160 hours withoutusing the partitioning membranes, the results obtained, when likewisedivided into five sections, were as shown in Table H, be-

Again, when a comparison is made of the yields (the number of grams ofoil formed in one hour per ampere), it can be seen that according to themethod of the present invention a practically constant yield of about3.0 g./ah. was regularly obtained, whereas in the control a yield low.of less than one half of this value could only be obtained, TABLEII.-CONTROL Section I Section II Section III Section IV Section V TotalMaterial:

Material monoester (g.) 1, 900 l Material monoester replenished (g.) 1,006 2, 046 1, S21 10, 181 Reaction Conditions:

Reaction time (in) 20 35 35 160 Amount of electric current (ah.). 400700 700 700 700 3, 200 Current density (ah/rim?) S0 S0 80 80 80 OilFormed:

Rate of formation (g./ah.) 1. 59 1. 50 1. 24 0.93 O. Oil formed (g) 6361, 050 865 650 664 3, 865 Mcthyl-Focteuoate (g.) (23.6%) 215 (20. 5%)180 (20.8%) (23.8%) 159 (23.9%) 859 Methyl-S-oxyoctanoate (g). 163(25.7%) 240 (22.9%) 192 (22.2%) 184 (28.3%) 194 (29. 2%) 973 Dimetliylthapsiatc (g.) 323 (50. 7%) 495 (47. 1%) 385 (44. 5%) 228 (35. 2%) 225(34. 0%) 1, 656 Y 1gligh boiling fractions (g.) 10D 9.5%) 103 (12.5%) 82(12.7%) 86 (12.9%)

Monoester consumed (g) 5, 700 Oil formed/monoester consruned (percent)67.8 Dirnethyl thapsiate/monoester consumed (pen cent) 29.0 Yield(percent of theory) 37.2

As is apparent from a comparison of the foregoing Tables I and II, itcan be seen that when the operation was continuously carried out for thesame number of hours under identical conditions, in case of the control(Table II) (the method without the partitioning membranes) a productionof only 3,965 grams of oil was obtained, whereas according to the methodof the present invention (Table I) a production of 9,524 grams of oilwas obtained and moreover with a reaction efliciency that is markedlysuperior. In addition, the amount occupied by dimethyl thapsiate, theintended diester, in the oil formed was about 65% by weight according tothe method of the present invention in contrast with about 50.7% byweight in Section I according to the control experiments illustrated inTable II. Further, as is apparent from a comparison of Sections II-V oftwo methods, according to the method of this invention (Table I) theamount formed in each section is practicaliy constant, showing that thereaction proceeds regularly, whereas in Table II (control) the amountforced decreases with the passage of time, showing that the reactiondoes not proceed regularly. It can also be seen that while it isexceedingly disadvantageous to carry out the operation continuously formore than about 60 hours according to Table II (control), according tothe method of this invention (Table I) even after operating for hours,it still is possible to obtain the product regularly in good yield.

this yield moreover being not only nonuniform but also manifesting amarked decrease with the passage of time.

It is also apparent that the formation of the undesirable high boilingfractions is of such a very minute amount according to the method of thepresent invention that it can be substantially ignored, whereas in TableII (control) the formation of the high boiling fractions is aconsiderable amount making it not ignorable, the proportion (showntogether in the table as a percentage) occupied in the oil formedincreasing with the passage of time. When a comparison is made inparticular as regards the formation of impurity in the form of the oxyacid, according to the method of this invention it was about 20% atmost, being usually about 17%, whereas in Table 11 (control) it reachedabout 29%, showing moreover a tendency to increase with the passage oftime.

As is apparent from the fore oing results, it can be seen that sinceaccording to the method of the present invention the reaction is carriedout regularly in good yield, the method is operable under constantelectric power conditions. On the other hand, in the method illustratedin Table II (control), in order to obtain the production of oil incommercially profitable amounts, operation under constant powerconditions is impossible, it being necessary that the voltage andelectric power be increased with the passage of time. Especially, whenthe operation is carried out by the method without the partitioningmembranes and moreover by the batch process as has actually beenattempted heretofore, with the requirement for a still greater voltageand electric power progressively with the passage of time, commerciallysatisfactory results can hardly be obtained.

In addition, as regards the oil formed in the anodic liquid inclusive ofthe diesters that are solubilized and accumulated therein, when theamount accumulated are shown by sections as in the foregoing Tables Iand 11, they are as in the following Tables I and II, respectively.

TABLE I 1 Section I Section II I Section III l Section IV Section V 1Quantity of reaction liquid (g) S. 780 10. 000 10. S90 10. 050 9, SQuantity of oil formed (g.) 280. 381. 2 309. 9 328. 3 324. 5Concentration in liquid (percent) 3. 19 3.81 '2. 64 l 3. 2S 3. 31

TABLE II.C ONTROL Section I Section II Section 111 Section IV I SectionV Quantity of reaction liquid (g) c. 7, 450 8, 900 10, 930 11, 070 10,100 Quantity of oil formed (g) c. 149 355. 8 467.1 669. 0 815. 7Concentration in liquid tpcrcent). 2.00 4. G0 4. 27 G. 04 l 8.06

As indicated by the corresponding Sections IIV of the foregoing twotables, i.e., Tables I and II, the nronortion occupied in the anodicliquid of the oil that accumulates therein is low and practicallyconstant in each of the sections as compared with the case of thecontrol which shows a marked progressive increase with the passage oftime. This brings about an increase in the resistance of the liquid andbecomes one of the causes for the requirement for high voltage and highelectric power. Again, the phenomena in the control of an increase inthe high boiling fractions (see Table II) and an increase in the oilaccumulated (see Table II) results from the formation of alkali in thevicinity of the cathode, as already mentioned hereinbefore.

Example 2.A water-methanol solution (concentration of methanol 10% byweight) of the same composition, same concentration and same pH was usedinstead of the aqueous solution of Example 1. Except that a directcurrent of 10-11 volts was used while replenishing the anodiccompartment with methanol to compensate for the minute quantity of themethanol which moves to the cathodic compartment, otherwise the sametechniques were followed as in Example 1. After a reaction time of 150hours, dimethyl thapsiate was likewise obtained satisfactorily. Theyield was 59% of theory.

Example 3.-Except that a water-methanol solution whose concentration ofmethanol was 50% was used and a direct current of 13-15 voltswas used,otherwise the same procedures were followed as in Example 2, Whereby wassatisfactorily obtained likewise after a reaction time of 50 hoursdimethyl thapsiate, the yield of which was 62% of theoretical yield. Ina case such as this where the concentration of methanol is high, the oilformed containing the diesters does not rise to the top but forms ahomogeneous layer. Consequently, instead of the oil and water separatingtank 4 a continuous extractor using hexane as the extracting agent isemployed, and the oil formed is transferred to the hexane. Theextracting agent may be any lower chain hydrocarbon that issubstantially insoluble in methanol, the preferred being hexane. Thisoperation is unnecessary when the concentration of methanol is so lowthat the oil operation performed is immiscible when the concentration ofmethanol is so low that the oil portion formed is immiscible, suchoperation is unnecessary.

Example 4.When the same techniques as in Example :1 were followed,except that an aqueous solution of the centration was 30% and having apH of 7 was prepared using the potassium salt of methyl hydrogenbrassylate and the potassium salt of methyl hydrogen Z-methyl succinatein a proportion of 1 mol of the former to 1 mol of the latter, and thenthe reaction was carried out for 30 hours at 9-11 volts and 20 amperes,whereby was obtained likewise satisfactorily dimethyl3-methyl-n-tridecane dicarboxylate.

Having thus described the invention, what we claim is:

1. A method of preparing a dialkyl dicarboxylate which comprisesconverting an alkali salt of said dicarboxylate into a diester by meansof electrolytic oxidation, in a cell consisting of at least one anodiccompartment and one cathodic compartment and a cationic exchangemembrane, feeding a solution of said alkali salt of an alkyl hydrogencarboxylate having a pH of 6 to 7 and a concentration of 20 to 40% byweight to said anodic compartment, maintaining said cathodic compartmentin an alkaline state and having an alkaline concentration of 0.5 to 3%by weight and passing an electric current through the system to efiectelectrolytic oxidation.

2. A method of preparing a dialkyl carboxylate which comprisesconverting an alkali salt of said dicarboxylate into a diester by meansof electrolytic oxidation, in a cell consisting of at least tireecompartments using at least two cation exchange membranes, making thecompartments on the two ends cathodic and the compartment adjacentthereto anodic, thus having cathodic and anodic compartments inalternation, feeding to said anodic compartment a solution of saidalkali salt of an alkali hydrogen dicarboxylate having a pH of 6 to 7and a concentration of 2G to 40% by weight, maintaining said cathodiccompartments in an alkaline state and having an alkali concentration of0.5 to 3% by weight and passing an electric current through the systemto effect electrolytic oxidation.

3. A method of preparing a dialkyl dicarboxylate, which comprisesconverting an alkali salt of said dicarboxylate into a diester by meansof electrolytic oxidation, in a cell consisting of one anodiccompartment and a cathodic compartment and an anionic exchange membrane,passing an electric current through the system, continuously feeding asolution of said alkali salt of an alkyl hydrogen dicarboxylate having apH of 6 to 7 and a concentration of 20 to 40% by weight to said anodiccompartment and water to said cathodic compartment containing analkaline solution having an alkali concentration of 0.5 to

13 3% by Weight and concurrently continuously taking a liquid phasecontaining the formed diester from the anolyte separating the diester,continuously recycling the residual liquid to the anodic compartmentafter adjusting the concentration and pH thereof, and continuouslydischarging the cathodic compartment liquid.

4. The method of claim 1, wherein said electrolytic oxidation is carriedout by using a solution of an alkal salt of an alkyl hydrogendicarboxylate having a pH of from 67, a concentration of 20-40% byweight, and employing platinum as the anode.

5. The method of claim 1 in Which said solution of an alkali salt is asolution selected from the group consisting of Water and Water-solublelower aliphatic alcohols.

6. The method of claim 1, wherein the alkalinity of said cathodiccompartment at the start of the electrolysis is maintained by means of a0.53% by Weight dilute aqueous solution of a substance selected from thegroup consisting of caustic soda, caustic potassium hydroxide,

sodium carbonate, sodium bicarbonate, potassium carbonate and potassiumbicarbonate.

References Cited by the Examiner UNITED STATES PATENTS 2,439,425 4/ 1948Gresham 20472 2,680,713 6/1954 Lindsey et. a1 20472 2,867,569 1/1959Kronenthal 204-72 OTHER REFERENCES Glasstone et al.: ElectrolyticOxidation and Reduction: Inorganic and Organic. New York, D. VanNostrand Co. Inc., 1936, p. 308309.

JOHN H. MACK, Primary Examiner. HOWARD S. WILLIAMS, Examiner. H. M.FLOURNOY, Assistant Examiner.

1. A METHOD OF PREPARING A DIALKYL DICARBOXYLATE WHICH COMPRISESCONVERTING AN ALKALI SALT OF SAID DICARBOXYLATE INTO A DIESTER BY MEANSOF ELECTROLYTIC OXIDATION , IN A CELL CONSISTING OF AT LEAST ONE ANODICCOMPARTMENT AND ONE CATHODIC COMPARTMENT AND A CATIONIC EXCHANGEMEMBRANE, FEEDING A SOLUTION OF SAID ALKALI SALT OF AN ALKYL HYDROGENCARBOXYLATE HAVING A PH OF 6 TO 7 AND A CONCENTRATION OF 20 TO 40% BYWEIGHT TO SAID ANODIC COMPARTMENT, MAINTAINING SAID CATHODIC COMPARTMENTIN AN ALKALINE STATE AND HAVING AN ALKALINE CONCENTRATION OF 0.5 TO 3%BY WEIGHT AND PASSING AN ELECTRIC CURRENT THROUGH THE SYSTEM TO EFFECTELECTROLYTIC OXIDATION.